The present invention relates generally to structures for providing a stable platform at sea for such purposes as instrumentation support, navigation aids, mooring for ships, drilling platforms, energy extraction and aquaculture. More particularly, the invention relates to such a structure which is capable of surviving extreme environmental conditions, and which minimizes the risk of damage to itself and to vessels in the event of a collision. Structures embodying the invention are suitable for use in relatively shallow off-shore waters (e.g. thirty feet) where wave height can be a significant fraction of the total water depth, as well as in deep water applications.
As is well known in the art, there are significant problems involved in providing stable, fixed platforms at sea which can survive extreme sea conditions, such as hurricanes, thereby avoiding the cost and inconvenience attendant to the loss of what are relatively expensive installations. Ocean structures in shallow water in particular face different, and often more severe, conditions than either land or deepwater structures, yet they have almost exclusively been based on traditional designs evolved in those two regions. Environmental conditions may remain mild for extended periods, particularly in tropic and subtropic waters, until a major storm or hurricane occurs, subjecting the structure to intense breaking wave impact forces, high currents, and resulting foundation scour. As a further example of the differences, and based on personal experience of the inventor in designing and installing coastal structures, a much more likely danger is an encounter with a vessel. This more likely danger is a result of the increased number of vessels near shore and the concentration resulting from channel-bound traffic.
Traditionally, two general classes of structures have been employed to provide fixed platforms at sea. Structures of the first class are moored floating buoys. Structures of the second class are rigid, and range from a single piling driven into the sea floor, to tower-like steel structures used in offshore drilling and production. These two classes of structure are discussed in detail next below.
Moored floating buoys and other floating structures, which perform satisfactorily in deep water, show deficiencies as depth decreases. To briefly enumerate three:
(1) Imprecise positioning due to allowable watch circle;
(2) Violent wave induced motion in storms; and
(3) High incidence of mooring failure or dragging from cyclic impact loading in storms.
Thus, moored floating buoys, by their very nature, are relatively less stable than fixed platforms, and this factor alone can make them unsuitable for many applications. Navigational markers, in particular, require precise positioning, and high reliability, in spite of high collision incidence probability. A surface-following buoy is particularly poor in this regard.
Certain forms of buoys, however, provide much better stability. For example, spar buoys are relatively unaffected by wave motion (except in the resonant mode). On the other hand they are much more susceptible to current and wind induced tilting, particularly with any appreciable payload.
Moreover, one analysis of a tethered spar buoy Chabra, Narender K., "Dynamics of a Tethered Spar Buoy System-Validation Using Full Scale Ocean Tests Data," ASCE Proc. Civil Engineering In The-Oceans/4, Vol. 1, 1979, p. 208, refers to a parameter called the heave response amplitude operator, which can vary from less than 0.1 to 4.0 or even higher at values of nd between 0.2 and 0.5, where n is the wave number and d is the draft of the buoy. This analysis would indicate that a spar buoy drawing ten feet could impact the bottom in six foot waves having a period of from five to eight seconds, a likely wave condition in thirty foot deep water.
In any case, the survival rate of all types of buoys in extreme sea conditions has not been good. Small navigation buoys using anchors of over twice their displacement often break or drag their moorings in storms. Any buoyant object moored by a flexible, slack chain or cable imposes severe strains on the mooring due to the momentum of the object. In a breaking wave condition, a buoy will surf down the face of the wave with considerable velocity, until the mooring abruptly stops it short.
Another form of buoy which has been advocated is a taut moored buoy. This approach has proven successful to others in deep water, Schick, George B., "Design of a Deep Moored Oceanographic Station", Transactions, 1964, Buoy Technology Symposium", MTS, p. 119; Black, David L., "A Stabilized Buoy for Oceanographic & Meteorological Instrumentation", Transactions, 1964, Buoy Technology Symposium, MTS, p. 603; and Fofonoff, N. P., "Current Measurements from Moored Buoys", Transactions, 2nd International Buoy Technology Symposium, MTS, 1967, p. 409. Comparative tests have shown that taut moorings are best suited for survival in the open sea. An example of a taut mooring employing a submerged buoyant member is disclosed in Paull et al U.S. Pat. No. 4,110,628.
Thus, the taut moored buoy approach is not readily applicable to shallow water buoys. Many taut moored systems, in order to remove the body providing the "tautness" from the effect of waves, are completely subsurface or rely on a subsurface float as the mooring point. Others employ surface floats of various streamlined shapes (e.g. spar, plank on edge, disk, boat shaped or catamaran) to reduce buoy motion and the loading on the moorings. In deep water, where even the highest wave is small relative to the depth, the mooring line is unlikely to go slack as the buoy enters a trough, nor is the buoy likely to submerge under a crest. One cannot make the same assumption when designing for wave heights of two thirds the water depth as is the case in relatively shallower waters.
The second general class of structure mentioned above is a rigid structure. The rigid structure solves some of the problems of floating buoys, while creating others of its own. A simple example of a rigid structure is a single vertical piling driven into the sea floor, and such structures are employed as navigation markers in relatively protected waters. Rigid pile structures are best suited to supporting gravitational loads through compression, and are less suited to carrying side loads through shear. In a collision situation they can fail catastrophically, or result in damage to relatively smaller vessels, or both.
As a more rigorous example of the limitations of this relatively simple rigid structure, static analysis of a ten-inch diameter pre-stressed piling loaded by a ten foot wave in thirty feet of water, using methods outlined in the "Shore Protection Manual", Vol. II, pp. 7-81, U.S. Army Coastal Engineering Research Center, 1973, predicts failure of the piling at the mudline.
Dolphins are sometimes employed as mooring and fendering devices around harbors, and for supporting navigational aids in any waters. A dolphin is a cluster of essentially vertical, slightly inwardly angled, pilings, typically of wood driven into the bottom and penetrating above the surface. Generally, the pilings are banded together at their top ends. A group of pilings arranged in a dolphin could be designed to withstand a ten foot working wave and perhaps even a twenty-one foot design wave. Topped with a platform, such a structure looks much like a section of a pier cut off and transplanted to deeper water. Survival of ocean front piers in extreme sea conditions has not traditionally been good.
Further raising questions of the structural integrity of rigid structures, the Texas Coastal and Marine Counsel at Austin, Tex. constructed a 45-foot tripod type steel tower mounted to the deck of a scuttled liberty ship to mark the site as an artificial reef. This tower was constructed in response to repeated losses of large (565 cubic foot) free floating buoys. After service of little over a year, this tower was "laid over on the deck by unidentified forces.", Lee, Howard T., "Buoying & Marking of Artificial Reefs--A State Experience," Proceedings Artificial Reefs, Florida Sea Grant Report #41, February 1981. This experience is a typical example of the cost and frustration encountered in erecting shallow water platforms.
Even aside from the question of structural integrity, the logistics of erecting a pile-supported structure require a pile driver, with accompanying derrick barge and tug, all of which are costly.
An approach intermediate a moored floating buoy, on the one hand, and a rigid structure on the other hand, is an articulated column. An articulated column can be conceptualized as a rigid buoyant piling fitted with a zero moment pivot pin at its base, or as a spar buoy extended downward until only one link of its mooring chain is left. One example of such an articulated column is disclosed in an article by John L. Kennedy, "Buoyant Tower Would Allow Deepwater Platform Drilling"; The Oil and Gas Journal, Oct. 28, 1974, pages 61-67. Another examples is disclosed in Pogonowski et al U.S. Pat. No. 3,708,985.
An advantage of this configuration, due to its compliant nature afforded by the pivoting base, is its ability to withstand extreme wave forces, without experiencing the high velocities and momentum forces of the surface following buoy.
However, where the purpose is to provide a relatively stable platform which maintains its orientation under normal working wave conditions, such an articulated column falls short, as will be appreciated in view of the following analysis read in conjunction with the plots of accompanying FIG. 1.
FIG. 1 individually plots several relevant forces as a function of column diameter for an articulated column in water thirty feet deep. In FIG. 1, plot line B is total buoyant force of a hollow column, with buoyancy provided by air inside, neglecting the weight of the column. Plot line F.sub.B is the tangential component of the total buoyant force B at an angle of displacement .theta.=15.degree.. Four wave force functions, F.sub.M, are plotted for respective wave heights H of H=3, H=6 and H=8 feet. As indicated, the wave force F.sub.M is a function of .phi..sub.M, a parameter used to calculate wave forces on cylindrical bodies, W, the specific weight of seawater, which is 64 lb/ft.sup.3 ; C.sub.D, a drag coefficient; as well as wave height H and pile diameter D. The wave force function F.sub.m is taken from the "Shore Protection Manual" referenced above. Conservative estimates for the various parameters are employed in FIG. 1
Based on the plots of FIG. 1, it can be predicted that the restoring buoyancy of the column is less than the ten foot wave force up to column diameters D of 8.2 feet, assuming that a 15.degree. displacement from vertical is tolerable. In other words, for diameters less than 8.2 feet, the displacement would exceed 15.degree. under the force of ten foot waves. An 8.2 foot diameter is unrealistic because, even in thirty feet of water, the static mooring forces alone would exceed 100,000 pounds.
Thus, relying upon buoyancy for vertical stiffness is counterproductive. Moreover, since the restoring force is proportional to the sine of the deflection angle from vertical, the buoyant restoring force does not start holding the structure up until it is well on its way down.
Accordingly, the traditional approach to supporting vertical towers is to employ guy wires. This, however, turns the articulated column back into a rigid structure which relies on its weakest link (the guy wires) for support. As the guy wires attempt to maintain the tower vertical, extreme forces are developed as a result of the non-compliant nature of a rigid structure, and long-term survivability is unlikely.
A partial answer is provided by clump weight systems which include what would otherwise be relatively slack guy wires made taut by clump weights attached near the ends of the guy wires and designed to rest on the sea floor. The guy wire dimensions are adjusted such that the tower is held vertical when the clump weights are resting on the sea floor. Such a system is disclosed in Beck et al U.S. Pat. No. 3,903,705.
The compliancy thus provided increases the survivability of the tower. When sufficient force is applied, the clump weights are lifted off the bottom. Thus, while the tower is allowed to tilt, catastrophic failure may be averted. In an analysis of geo technical aspects of a compliant tower, Audibert et al states, "the flexible response characteristics of the tower and guying system significantly reduces stresses in structural members as it allows the tower to essentially move as a rigid body (Audibert, Jean; Dover, Anthony R., Thompson, Grant P., and Hubbard, Jack L., ASCE Proc. Civil Engineering in the Oceans/4, Vol. 1, 1979, p. 820.) By adjusting the weights, and thus the force, the response parameters such as compliance of the tower can be "tuned" or adjusted over a wide range of selected displacements.
The Audibert et al model referred to above is for a platform in relatively deep water where the allowable excursions are small, such as drilling platforms beyond the continental shelf. Such use of clump weights, however, does not appear to be a viable alternative for use in intermediate depth water, for two reasons:
(1) "Tuning" or adjusting the tower for high compliance allows corresponding large excursions of the clump weights. Limiting the compliance or stiffening the structure requires heavier weights. In either case, the inertial loading of the accelerating weights (which are themselves subject to complex wave forces totally out of phase with the tower loading) on the guy wires can be severe, particularly if opposing wires alternately go slack once each wave period.
(2) In an area of littoral suspension and resettlement, clump weights tend to become buried and thus convert to non-compliant moorings. This problem was addressed without solution by Audibert et al and is also addressed by the disclosure of Beck et al U.S. Pat. No. 3,903,705.
One of the above two reasons could be responsible for the failure of the Atlantic Oceanographic Laboratory Stable Platform, a guyed tower using clump weights installed in 187 feet of water near Halifax Harbor entrance.
According to an unpublished report, that particular structure performed well for several months in seas up to eight meters, but was damaged in ten to twelve meter waves when two guy wires parted. The failure was attributed to underestimation of the wave climate, underestimation of wave forces on the structure, occurrence of the structure's pitch resonance within the range of predominant wave energy, and inability to properly position and tension the clump anchor.
The Atlantic Oceanographic Laboratory Stable Platform was not constrained at its base. Rather, it relied on its net 5.9 tons of negative buoyancy and penetration into the sediment to eliminate heave. The assumption that heave would be so eliminated appears to be invalidated by evidence in the report, including lateral displacement of the tower, failure of the base section, as well as measurements from a vertical accelerometer mounted on the platform.
Variations on the clump weight approach have been proposed. For example, Miller U.S. Pat. No. 3,524,323 proposes a system of guy wires terminated at their lower ends by weights confined in vertical tubes to permit the weights to be vertically lifted off of the tube bottoms as the tower tilts.
In a more substantial variation, Borrmann et al U.S. Pat. No. 2,986,888 proposes a guy wire system with submerged floats, rather than weights, affixed to intermediate points on the guy wires. In order to maintain the floats in relatively fixed positions when the tower is in its undeflected vertical position, each float actually has a pair of anchor lines spaced apart at their lower ends to define a triangle with the float at the apex of the triangle. This anchor line configuration limits the upward movement of the Borrmann et al floats in a manner analogous to the manner in which the seafloor limits downward movement in a clump weight system.
Another disadvantage, common to both clump weight systems and guy wire float systems, is that there is an upper limit to the deflection before each weight or float is in line with its respective guy wire and the system reverts to a rigid guy wire system. Thus, past a certain tower tilt angle, the system loses its compliancy. It is precisely when compliancy is most needed for survivability, i.e. under extreme environmental forces, that compliancy is lost.
Yet another disadvantage is the large amount of area required for the guy wires and buoys or weights.